Avalanche Forecast
Issued: Dec 2nd, 2016 11:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Storm Slabs.
NWAC,
Northwest Avalanche Center
Watch for left over wind and storm slab along the west slopes on Saturday. Wind slab from Friday may get covered by less cohesive snow Friday night.
Summary
Detailed Forecast
West winds will slightly decrease but not let up on Saturday with mostly light snow showers much of the day expected along the west slopes and cooler temperatures. However alpine west winds and snow should begin to increase in the afternoon as the next system approaches.
Any wind slab from Friday seems likely to linger on Saturday. This is mostly likely on northwest to southeast slopes in the near and above treeline where snow from Friday will have been heaviest such as at Baker and Rainier. Watch for firm wind transported or hollow sounding snow. Wind slab from Friday may get covered by less cohesive snow Friday night making it harder to detect the underlying wind slab layer.
Storm slab may still be possible Saturday in any areas that had rapid accumulations of new snow with warming on Friday. Storm slab is generally shorter lived than wind slab but may last into Saturday along the west slopes in the near and above tree line.
Be prepared to change your plans if the next storm begins to arrive and significant snow begins to accumulate sooner than expected on Saturday afternoon.
In most of the lower part of the below treeline band there is still insufficient snow for avalanches but watch for early season hazards such as terrain traps, rocks and creeks. The shallowest snow is at pass level at Stevens, Snoqualmie and White Passes.
Snowpack Discussion
Weather Discussion for the Cascade West Slopes
A bit of a break was seen Thursday with snow showers west of the crest generally tapering with 1-2 more inches common along the west slopes ending Friday morning but with 5 inches at Paradise.
A front is crossing the area on Friday causing moderate to strong south to west alpine winds in the Olympics and Washington Cascades, several inches or better of new snow along the west slopes and a warming trend.
Recent Reports for the Cascade West Slopes
Generally stable conditions were indicated by reports Wednesday and Thursday.
NWAC pro-observer Lee Lazzara was on the west side of Mt Baker on Thursday up to about 5300 feet. He found 35 cm of right side up recent snow and saw no signs instability. Compression tests were negative for a wind layer within the recent snow. He could see some wind effects at higher elevations.
Reports from the Stevens pro patrol and from NWAC pro-observer Dallas at Stevens on Thursday also indicated generally stable, well bonded, right side up snow conditions.
The weather on Friday brought a change to the previous couple days.
NWAC pro-observer Simon Trautman was in the Bagley Lakes area near Mt Baker on Friday and at about 5000 feet found dangerous new wind slab conditions due to winds, new snow and the warming trend. He reported a very reactive easy to trigger 15-25 cm new wind slab giving cracking to 2-4 feet and up to 20-30 feet where the new wind slab was deeper. Low visibility prevented skiing in avalanche terrain.
Wind slab layer in a snow pit on Blueberry Hill in the Mt Baker back country on Friday. Photo by Simon Trautman.
The Alpental pro-patrol on Friday reported rain soaking into the snowpack in the below treeline band. They reported a small 2 foot climax avalanche on a rock slab due to the rain and warm temperatures.
Problems
Wind Slabs
Release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..
Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.
Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.
Aspects: North, North East, East, South East, North West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Storm Slabs
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Dec 3rd, 2016 11:00AM
